[Feature Request] Run headers through codec

As discussed on slack: by default, don't put headers through codec / custom data converter, except for otel baggage.

Putting otel baggage through the codec requires passing contextual info to Codec.encode/decode. And if we're going to do that, perhaps we want to allow users to have the codec be run on their other headers?

Additional context

For future reference, setBaggage returns a new context:

import { context, propagation } from '@opentelemetry/api';
    const baggage = propagation.createBaggage({ test: { value: 'true' } });
    const newContext = propagation.setBaggage(context.active(), baggage);

Start by tracing the Codec.encode/decode entry points and the header propagation path, then review how setBaggage creates the context. Establish tests for default header handling, OpenTelemetry baggage, and any opt-in conversion; done means the intended behavior is specified and covered.